December 23, 2008

Acid rain

The natural rainwater moving through atmosphere comes in contact with various chemicals produced and deposited in the atmosphere. These chemicals are produced by various natural (e.g. electrical nitrogen fixation due to electrical lightening), biological (e.g. release of gases in decay and decomposition of organic matter and other biological processes) and geological (e.g. volcanic eruptions and weathering of rocks) processes. Due to this contamination, natural rainwater in perfectly unpolluted areas is also somewhat acidic. The pH of normally clean or ‘pristine’ rainwater is generally agreed by scientists to be 5.6.If rainwater falling in an area has pH value below 5.6, it is called acid rain.

Recent measurements show that rain and snow having pH 4.3 or below fall regularly over many areas of heavily industrialized Northern hemisphere, specially North America, northern and western Europe. Sometimes individual storms under favourable conditions may have may have very low pH values. For example, in 1979, Kane in Pennsylvania, America recorded a rain of pH 2.7 and in same year, , Wheeling in West Virginia had rain of pH 1.5. In Britain, Pitlochry had a rainfall of pH 2.4 in 1974. The acid rains is caused by emission of large quantities of sulphur dioxide and oxides of nitrogen in the atmosphere due to burning of fossil fuel in various industrial and other activities of human beings. Allied to acid rains are phenomena of acid mist and acid fog, both of which come under the category of occult precipitation. The cause of acid mist or acid fog is high concentration of sulphates and nitrates in the form of fine aerosol particles (dust or soot) in wind-driven ground-level clouds which causes condensation of tiny water droplets around these particles. These droplets being tiny fraction of normal rain drops, do not fall as rain water but remain suspended in the atmosphere forming acid mist or acid fog.

The problem of acid rain has attracted worldwide attention only since 1980s. However, the term ‘acid rain’ was first used by first Alkali Inspector of Britain,, Robert Angus in 1872. His work largely remained ignored until 1950s when Canadian ecologist Dr. Eville Gorham undertook detailed studies of rainwater quality and its control in Lake District in north-west England. By mid 1960s, early damage symptoms of acid rains begun to appear in Scandinavia and Swedish worker Svente Oden begain a concerted scientific effort in 1967 to bring awareness about acid rain problem. He is considered to be the father of modern acid rain studies.

GEOGRAPHY OF ACID RAIN

Primary pollutants causing acid rain problem are blown over long distances by the wind and thus spreading the problem over whole of the Earth’s surface. However, till now most of pollutants responsible for acid rain problem are produced in the highly industrialized nations, the areas of the impact of acid rains are few, noticeable, few and predictable. Common properties observed in areas affected seriously by acid rain problem are:

Areas sharing the above common properties are termed acid rain hot spots and include many parts of Scandinavia, upland Britain, West Germany and many parts of Northern Europe. Across Atlantic, such areas include Nova Scotia, Canadian Shield around southern Ontario and Quebec, Adriaondack Mountains, Great Smoky Mountains, parts of Wisconsin and Minnesota, Pacific Northwest U.S.A., Colorado Rockies and Pine Barrens of New Jersey. Japanese islands are also included in this category.

In contrast to above areas, there are two types of safe areas where acid rains are not a problem at present. These areas include:

The areas located away from and not downwind of possible source areas and themselves having little polluting industrialization. These areas include almost all of southern hemisphere, tropics and parts of northern hemisphere e.g. northern Russia.

The areas that receive acid rains but have natural resistance to its damaging impact due to buffering capacity provided by the alkaline dust blown from the west. Actually alkaline rains have been reported in Sweden before 1960 in areas with limestone outcrops and cement manufacturing areas. Wind blown alkaline material can de derived from deserts (fine material brought over from Sahara and Gobi deserts has been reported), from wind erosion of top soil alkaline particulate pollutants e.g. soot from smoke-emitting chimneys and agricultural fertilizers.

Geologically, the areas most vulnerable to acid rains fall under three categories:

Areas having severest acid rain damage are glaciated Pre-Cambrian shield areas of Scandinavia, glaciated parts of upland Britain having thin soils, eastern Canada and resistant Canadian Shield and northwest U.S.A. Problems of acidification develop much acutely on granite and similar other resistant rocks.

Acid rain as global problem

Though at present acid rain problem is mainly concentrated in highly industrialized areas, the long-range transport of concerned air pollutants results in gradual globalization of the problem. As a result of slow transport of acid rain causing pollutants from heavily industrialized areas to areas till now free from this problem, the latter areas are also beginning to show acidification damage. Such damage has been reported from many developing nations like Zambia, South Africa, Malaysia, Venezuela, India and China. Most productive farmlands of China and India, paddy fields of South-east Asia and forests of Amazon in South America have soils which are highly susceptible to acidification.

Global dimension of acid rain problem was established beyond doubt in 1981 with discovery of Arctic haze. It is bluish-gray haze developing in Arctic areas similar to that frequently found over and downwind of large industrial areas in western Europe and eastern North America. Haze layers often cover a horizontal area of upto 1000 km and are caused by scattering of solar radiation by minute suspended particles in the atmosphere. These particles vary in the size range of 0.1-1.0 micrometer and mostly comprise of sulphate aerosols. These aerosols are transported by jet streams in upper atmosphere and may reach upto 8000 km away from their industrial sources. Hazes are found to be thickest in Alaska’s North Slope extending atleast to Norway. Hazes mainly affect visibility and are not as damaging as the smog.

CAUSE AND FORMATION OF ACID RAIN

SO2 and oxides of nitrogen (NOx) emitted into the atmosphere due to industrial, commercial and other anthropogenic activities are the basic cause of acid rain formation. Therefore, the problem of acid rains has accompanied the rise of emission of these gases into the atmosphere.

SO2 is emitted from three principal man-made sources:

Combustion of coal produces about 60% of total SO2 emitted into atmosphere.

Combustion of petroleum products which adds 30% of total emission.

Industrial activities like smelting of iron, zinc, nickel, copper ores, manufacture of sulphuric acid and operation of acid concentrators in petroleum industry. These produce the remaining 10% of this gas.

Overall emission of oxides of nitrogen is small in comparison with SO2, their importance in formation of acid rains is very high. Most of the oxides of nitrogen (NO3, NO2, NO etc.) are produced from:

Combustion of fossil fuels.

Industrial chimneys and thermal power stations.

Motor vehicles in urban areas.

Man-made sources of SO2 and NOx emission are point sources (e.g. thermal power stations and industrial chimneys) and the emission from these occurs as a plume of gases. The plume of gases emitted from high stacks usually travels downwind for about 12 km as a straight line without much dispersion. Afterwards, its shape evolves by diffusion and changes progressively downwind into a widening cone. The direction, speed, distance of travel of the plume and its dispersal and diffusion depend upon meteorological conditions such as direction, velocity and pattern of propelling wind, air temperature (especially the vertical temperature gradient), air turbulence and atmospheric stability. Under stable atmospheric conditions, for example, at night over land and during day over snow covered ground, there is very little vertical dispersal for very long distance and the acidification may occur at quite far away place from the source of emission.

Dispersal of the plume of SO2 and NOx occurs in the mixing layer of atmosphere that extends from ground level upto 1-2 km altitude. The dispersal is triggered by diffusion and atmospheric turbulence, normally between 5 to 25 km from the point of source. The rate of diffusion and mixing of oxides into air is faster when flow of air is turbulent. The lower portion of the dispersing cone of oxide plume first touches the ground level at about 5 km distance form the point source while middle and upper portions are thoroughly dispersed in the air leading to dilution and chemical transformation.

The deposition of pollutant oxides from the plume onto the ground is of two types: dry deposition and wet deposition.

Dry deposition: The acidic oxides deposited from the bottom of the plume between 5-25 km from the source in the form of gases and particles constitute the dry deposition. Though such deposition is not acid rain in strict sense, it produces acidification of soils and surface water bodies similar to acid rain. This dry deposition also causes direct SO2 and NOx poisoning of the vegetation. Dry deposition of sulphur and nitrogen oxides and undissolved acids on lakes and steams straightaway dissolve in the water and acidify the water bodies. Such dry deposition on land and on vegetation remains inactive till dew or rainfall when these dry deposited acids dissolve in the dew or rain water and form active acids. Such sudden addition of high concentration of acids into an otherwise stable environment causes acid shocks, acid flushes or acid surges. These terms indicate increasing levels of acidification and decreasing time period in which such acidification takes place. During winters, SO2 and NOx pollutants are dry-deposited on snow and ice in the catchment areas of many lakes and rivers. In the following spring season, when this snow and ice melt, the acids accumulated in the snow and ice over long period are suddenly released over a period of few days to a week causing acid surges in the lakes and streams.

Wet deposition: It is the deposition of acidic oxides of the plume over land or vegetation after being dissolved in the rainwater, snow or ice forming acid rains, acid snow, acid mist or acid fog. Today’s industrial chimneys are normally 100-300 meters high and, therefore, such wet deposition normally occurs beyond 25 km from the point source. The prevailing wind pattern and the length of time over which oxides are transported in the wind system is of great importance in the geographical distribution of acid rains. Longer the SO2 and NOx remain in the atmosphere, greater is the possibility of their transformation to produce sulphuric and nitric acids.

The practice of increasing the height of chimneys and installation of electrostatic precipitators to reduce the air pollution appears to have magnified the problem of acid deposition in two ways. Firstly, tall stacks of pollutant-emitting units now emit pollutant gases at much greater heights so that these gases are now dispersed over much wider areas increasing the geographical extent of acid deposition. Secondly, installation of electrostatic precipitators and other mechanisms to remove alkaline particulates in chimneys has resulted in increased emission of acidic gases. It is because prior to installation of such mechanisms, acidic gases were neutralized to a large extent by alkaline particulates being emitted alongwith them.

CHEMISTRY OF ACID RAINS

Strictly speaking acid rain is a term which indicates a wide variety of mixtures of acids and oxides in the rainwater. For example, rainwater of pH 4.5 may contain a high sulphur content, high nitrogen content or any combination of the two. Acidity of rainwater results from chemical transformations of a large number of acidic ions added to the atmosphere from natural sources (e.g. sea salts, volcanic emissions, biogenic emissions, soil etc.) and by human. Major such ions can be categorized as following:

Inorganic ions: These include trace metal ions which often act as catalysts to quicken the acidity processes. At coastal sites, corrections for the impact of seawater on rainwater quality have to made before accurate assessment of the role of land-based sources can be made. In individual locations, rainwater quality may be strongly influenced by local sources.

Organic ions: These are important alongwith local biogenic sources in affecting the precipitation quality, particularly in tropics.

The steps involved in each chemical process contributing to rainwater acidity depict a multitude of pathways with many of the steps being reversible and many of the steps exhibiting highly complex chemistry. Thus the overall chemistry of acid rain is extremely complicated because of the very large number of chemical interactions involved. Moreover, exact chemical composition of acid rain is not same in every area. It varies from place to place depending upon the proportion of different oxides present and the chemical transformations they have undergone during their stay in the atmosphere. Although a variety of natural and man-made oxides contribute to rainwater acidity through variety of chemical pathways, most important pathways are those associated with two major acidic gases i.e. SO2 and NO2 added to atmosphere from various polluting sources. The complex pattern of acid deposition has following six stages:

The atmosphere receives SO2 and NOx from natural and man-made sources.

Some of these oxides fall on the ground as dry deposition within 5-25 km from their parent sources.

Formation of photo-oxidants like ozone, is stimulated in the atmosphere.

The photo-oxidants interact with SO2 and NOx to produce acids (H2SO4 and HNO3) by oxidation.

The oxides of sulphur and nitrogen, photo-oxidants and other gases (including NH3) dissolve in the cloud and rain-droplets to produce acids (H+ and NH4+) and sulphates (SO42-) and nitrates (NO3–).

The most important step in this chain of reactions is the catalytic conversion of SO2 and NOx. This may take from a few hours to a few days in the atmosphere and can not occur without photo-oxidants (precurssors). Ozone is the most readily available and abundant photo-oxidant in the atmosphere . Hydrocarbons and NO added to the atmosphere as pollutants are the two main precurssors of ozone. The acid rain is the final product of the loading of SO2 and NOx coupled with photochemistry and physical dynamics of stratosphere.

Acid gases like SO2 and NOx are transformed into dilute acids in the rainwater by following three major types of reactions:

Homogeneous aqueous-phase reactions: These occur between individual species in a liquid medium such as cloud or raindrop.

Heterogeneous aqueous-phase reactions: These occur during adsorption of acid gases on solid surfaces and are extremely complex. These reactions probably assist in creating rainwater acidity but are not considered to be as important as other two types of reactions in the overall chemistry of acid rains.

The relative importance of any chemical process operating in the atmosphere depends strongly on the meteorological conditions such as the presence of clouds, relative humidity, intensity of solar radiation, temperature etc. Following two factors are crucial to the operation of each process:

Time available to complete secondary chemical reactions.

Availability of excited ions and catalysts to assist the reactions.

Homogeneous gas-phase chemistry

In dry atmosphere, most of the acid gas reactions leading to formation of acid ions such as sulphates and nitrates involve excited molecules, atoms, free radicals and sunlight. The OH radical is particularly important in such reactions. Following main such chemical pathways lead to eventual formation of sulphuric and nitric acids in rainwater:

SULPHUR DIOXIDE

Very slow reaction:

2SO2 + O2 ——- 2SO3

Unstable compounds:

OH + SO2 + M —– HOSO2 + M

HOSO2 + O2 —- HO2 +SO3

(M = catalyst; often Fe3+ or Mn2+)

Very fast reaction:

SO3 + H2O — H2SO4

NITROGEN DIOXIDE

Very slow reactions: ( ppb concentrations are reached in many days)

2NO + O2 — 2NO2

or,

HO2 + NO —– OH + NO2

2NO2 + H20 —— HNO3 + HONO

Factors affecting homogeneous gas-phase reactions

Interfering substances:Oxidation of SO2 and NO2 in the atmosphere is relatively a slow process and there may be several substances causing interference along the way. For example, HOSO2, which is a very unstable substance, may react with CO, NO, water vapour, various hydrocarbons and other chemical species and block the reaction described above.

Catalysts: The reactions between SO2 and NO2 with O2 in the dry atmosphere are considered to be so slow without catalysts that the eventual output of acid is very small. Reactions with the addition of catalysts and free radicals are the main sources of ions leading to acidity of rainwater.

OH radical: Oxidation rates of SO2 and NOx in a cloud-free atmosphere are highly variable and strongly dependent on the concentration of OH radical. If concentration of OH radical is relatively high (on the order of 9×106 mol cm-3), oxidation of SO2 to SO42- is approximately 3.7 +/– 1.9 % per hour. Conversion of NO2 to HONO2 is much more rapid reaction; its rate being about 34 +/– 17% per hour. With lower OH concentrations, the conversion rate is reduced and SO2 converts at a rate of about 0.7% per hour or about 16.4% per day. NOx conversion is at much faster rate and the rates vary between 6.2% per hour and 100% per day. In winters, conversion rates are 0.12% and 1.1% per hour respectively. At night, when OH concentrations are at minimum, conversion rates are sharply reduced.

Homogeneous aqueous-phase reactions

The species of sulphur and nitrogen can be incorporated in liquid water droplets in several ways e.g. (I) they may have high solubility in water; (ii) they may attach through diffusional processes; (iii) they may be incorporated through impactations and collisions and (iv) acid aerosol species may act as nuclei for formation of water droplets. Most important aqueous –phase reactions in acid-rain chemistry are as following:

A. SULPHUR DIOXIDE

SO2 + H2O <—- SO2.H2O

SO2.H2O — H+ + HSO3–

HSO3– ——— H+ + SO3–

O2 + 2HSO3– ——- 2H+ + 2SO42- (Reaction slow without catalyst)

H2O2 + HSO3– ——– H+ + SO42- + H2O (Reaction is rapid)

O3 + HSO3– —— H+ + SO42- + O2 (Reaction is rapid if pH>4.5)

B. NITROGEN DIOXIDE

NO2 + O3 —– O2 + NO3

NO3 + NO2 + M <—— N2O5 + M (M = Catalyst; often Fe3+ or Mn2+)

N2O5 + H2O ——- 2H+ + NO3 + NO2

Factors affectinghomogeneous aqueous-phase reactions

Reaction medium: Conversion of acid ions is much faster when reaction medium is water. At the droplet scale, sequence of conversions might be:

Initial diffusion of gas to the droplet interface.

Transfer across the interface into the droplet.

Swift aqueous-phase equilibrium.

Aqueous-phase reactions and concurrent diffusion.

Catalysts: In liquid water, catalysts are very important in determining the speed of conversion process. Models using proper chemical conversion estimates indicate that, with the exception of H2O2, impact of other catalysts is highly dependent on the pH level in the water. If pH of the droplet is of the order of 5.0, then conversion rates are significantly increased in the presence of O3, Fe3+, Mn2+ and other ions. However, at pH level of 4.5, trace metal ions contribute only about 1% per hour to the conversion process and the impact of ozone drops to about 10% per hour. At pH level of 4.0, trace metal ions have negligible impact and ozone adds only about 1% per hour to conversion process. This occurs because, in part, solubility of SO2 in water decreases with increasing H+ concentration.

Hydrogen peroxide: It enhances the rate of conversion of SO2 to SO42- independently of the pH level in water droplets. H2O2 dominates the aqueous chemistry process and may increase the conversion rates to 100% per hour depending upon the cloud type, altitude and other meteorological conditions until it is fully exhausted. Afterwards, ozone becomes the dominant catalyst of conversion reactions. H2O2 is not important in the formation of NO3–. Favourable conditions for the formation of H2O2 are low NOx concentrations and high concentrations of hydrocarbons and aldehydes in the atmosphere. The conditions favourable for ozone formation are unfavourable for H2O2 formation.

Cations in solution: The rates of formation of SO42- and NO3– may be altered by cations in solutions, particularly by ammonium (NH4+). The cations may increase the rate of oxidation of SO2 by more than an order of magnitude. Ammonia can dissolve as a gas in water droplets and thus directly reduce the rainwater acidity. Presence of extra cations enhances the impact of catalysts, especially at pH above 4.5. This results in formation of disproportionately high amounts of SO42- and NO3– in presence of cations in solutions than in presence of free H+. Ammonium seems to increase the formation of SO42- most in spring when concentrations of both NH4+ and H+ are highest. It has been suggested that about 50% increase in NH4+ in Europe since 1950s may have had some impact on the change in SO42- in rainwater relative to H+. If soil dust rich in cations like Ca2+ and Mg2+ is loaded into the atmosphere, these cations neutralize the strong acids and the rainwater tends towards alkalinity. For example, in India, strong acidic ions in atmospheres around urban areas are heavily neutralized by such soil dust.

Formation of NO3–: In areas where concentrations of hydrogen peroxide and ozone are negligible, formation of NO3– can control the production of sulphuric acid in atmosphere. The H2O2 is not important in the formation of NO3–. Though very little is known about conversion of NOx in aqueous environment, N2O5 is supposed to play important role and perhaps of NO3– is directly formed from it depending on the relative concentrations of NOx and NO3–. In the night, reaction of oxides of nitrogen with ozone can produce significant amounts of NO3– because of the absence of its photochemical destruction.

Season and time of day: Season and time of the day have important impact on acidity of rainwater and cloud-water due to following important reasons:

Difference in pollutants and ions: There are generally different mixtures of pollutants and ions available for acid conversion at different times of day and in different seasons.

Difference in gas-phase reaction rates: In winter, available solar energy is weaker and, therefore, gas-phase chemical reactions are slower than in summer.

Difference in concentrations of catalysts: In winters, oxidation in clouds generally decreases because concentrations of appropriate catalysts are lower than in summers. For example, levels of H2O2 may be about 16 times higher in summers (about 4.8 ppbv) than in winters (about 0.3 ppbv). This high H2O2 concentration in summers enhances the formation of SO42- in that season. At night, conversion of SO2 to SO42- may reach 10% per hour in good catalytic conditions such as low stratus clouds over water.

Difference in photochemistry: The photochemical production and destruction of chemical species in atmosphere depends on the availability and intensity of solar radiation and, therefore, may affect their concentrations during day and night. For example, concentration of NO3– increases considerably at night when it is not being destroyed photochemical reactions.

Types of clouds and precipitation: Mechanism of removal from the clouds may vary by the types of clouds and precipitation. In the clean background air of southern hemisphere, gas-phase and aqueous-phase reactions are almost equal in importance. However, in northern hemisphere, particularly in winter season, aqueous-phase reactions become dominant.

Chemistry of acid fog

More recently, measurements at sites in parts of Europe, California and eastern U.S.A. have shown that in most circumstances, acid fog and water in low clouds has a lower pH value than equivalent acidic rainwater. Average pH values of acid fog in areas of heavy air pollution are about 3.4 and range from 2.8 to over 5.0 On average, mean concentrations of H+ and acid ions are 3 to 7 times higher in fog-water than in equivalent rainwater. Acid fog-water also has higher concentration of anions and cations. There are following five main reasons for the above describe differences:

Fog being located nearer to the ground, is often exposed to higher pollutant concentrations for longer periods of time than the rainwater during below-cloud scavenging. This exposure allows more time for extensive aqueous-phase chemical processes to take place.

The smaller droplets in fog and mist have a greater combined surface area compared to raindrops. As a result, acid gas diffusion is enhanced and higher concentrations of the resultant ions are produced.

The fog remains in the air mass in which it is formed while precipitation is often associated with changing air masses in frontal situations when much of the gaseous and aerosol material in the atmosphere is removed.

Pollutant aerosols originating several hundred kilometers away often act as nuclei for fog or cloud droplets and enhance aqueous chemical processes. The size and number of water droplets formed and the resultant chemistry depend on the number of aerosol nuclei available in the cloud. Greater number of these generally produce smaller and more numerous fog droplets. Ion concentrations in mist tend to be lower than in fog because mist contains a lesser number of droplets and this limits the chemical reactions.

However, fog-water shows wide variations in ion concentrations between sites and events. In stable atmosphere, low altitude fog masses are more likely to interact with pollutant emissions near the surface e.g. NOx from automobiles. On the other hand, mountain fogs occurring in a well mixed atmosphere and at times, isolated from low-altitude pollutant emissions due to inversions, tend to be cleaner having pH values of 5.0 and above. On minor scale, dew from polluted atmosphere can also be acidic with free H+ comprising about 80% of acidity while species of sulphur and nitrogen may contribute about 60% and 30% respectively to the acidity.

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Fascinating! I have been writing a blog for about a year now having discovered (to my horror) that trees are failing at a rapidly accelerating rate. I am very interested in the role of pollution and acid rain.

I would very much like to know what sources you use for your analysis of the effects of acid rain. Please visit my blog & leave a comment or write directly to witsendnj@yahoo.com! Thank you so much!
Gail
New Jersey, USA